Photovoltaic device

ABSTRACT

This invention is a layered thin film semiconductor device comprising a first transparent layer; a thin, second transparent layer having a conductivity less than the first transparent layer; an n-type layer; and a p-type layer comprising one or more IIB and VIA elements. This invention is also a method for making such semiconductor device. The thin film semiconductor devices of this invention are useful for making photovoltaic devices.

This application claims the benefit of Provisional Patent ApplicationNo. 60/289,481, filed on May 8, 2001.

FIELD OF THE INVENTION

This invention relates to a new, thin film semiconductor device. Moreparticularly, this invention relates to a new thin film photovoltaicsemiconductor device having improved efficiency in converting solar orother light energy into electrical energy. This invention also relatesto a method of manufacturing such semiconductor and photovoltaic device.

BACKGROUND OF THE INVENTION

Solar power is an important source of renewable electrical energy. Thecontinuous challenge in the field of solar energy is to develop andmanufacture photovoltaic devices having a high efficiency for convertingsunlight into electrical energy. The more efficient the photovoltaicdevice is at performing such a conversion, the greater amount ofelectricity can be generated for a given investment. While a number ofdifferent types of photovoltaic devices have been developed, aparticularly suitable photovoltaic device is a thin film semiconductordevice having at least one layer, a p-layer, comprising one or more IIBelements and one or more VIA elements from the Periodic Table ofElements. One such photovoltaic device is referred to as a CdTe devicebecause the IIB and VIA elements are cadmium and tellurium,respectively. Typically, these photovoltaic devices also have an n-layeror window layer generally comprising cadmium sulfide. Such a device issometimes referred to as a CdS/CdTe device or cell. They also typicallyhave a transparent, electrically conductive first contact and a second,generally opaque, electrically conductive second contact. In a usualconfiguration, these devices have a first transparent conductive layerof conductive metal oxide which is a first electrical contact, ann-layer or window layer of the n-type comprising cadmium sulfidedeposited on the first transparent conductive layer, a p-layer depositedon the n-layer and a second, generally opaque electrical contactdeposited on the p-layer. The junction of the n-layer and the p-layer isa heterojunction, as is known in the art, and is responsible for thegeneration of electric potential and electric current when thesemiconductor device is exposed to light energy, such as sunlight. Lightenters the device from the side of the first transparent layer. Suchdevices have demonstrated superior efficiency and power generationcompared to other types of thin film photovoltaic devices. See forexample the article by D. Cunningham et al., “Large Area Apollo ModulePerformance and Reliability,” 28^(th) IEEE Photovoltaic SpecialistsConference, Anchorage, Ak., September 2000. However, while such CdTethin film, semiconductor photovoltaic devices are efficient and are alsoamenable to commercial manufacturing methods, the art needs suchphotovoltaic devices with improved efficiency. The present inventionprovides for such thin film, semiconductor photovoltaic devices havingimproved efficiency in converting sunlight into electric current.

SUMMARY OF THE INVENTION

This invention is a layered thin film semiconductor device comprising afirst transparent layer; a thin, second transparent layer having aconductivity less than the first transparent layer; an n-type layer; anda p-type layer comprising one or more IIB and VIA elements. Thisinvention is also a method for making such semiconductor device. Thethin film semiconductor devices of this invention are useful for makingphotovoltaic devices.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the layered structure of one of the embodiments of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention comprises a layered, thin film semiconductor devicesuitable for generating electrical current upon exposure to lightenergy, particularly solar energy. In one embodiment, the inventedlayered, thin film device comprises a first transparent layer comprisinga conductive material, a thin second transparent layer having aresistivity more than the resistivity of the first transparentconductive layer, an n-type layer and a p-type layer; the p-type layercomprising one or more IIB and VIA elements of the Periodic Table ofElements. As used herein, the term n-type means a negatively dopedsemiconductor and the term p-type means a positively dopedsemiconductor. In a preferred embodiment, the layered, thin filmsemiconductor device is supported by or deposited on a suitablesubstrate material such as a glass, plastic or metal. Preferably thesubstrate material is glass, preferably a clear glass and preferably theglass is in the form of a flat sheet.

The substrate material is most preferably clear, flat glass, and canhave any shape but is generally square or more preferably rectangular.The thickness of the glass can be any thickness that provides for thenecessary support of the thin film semiconductor device but generally isabout 0.2 to about 5 millimeters in thickness.

Preferably, the first transparent layer comprises one or more conductivemetal oxides such as tin oxide, zinc oxide, indium tin oxide, or amixture of one or more of these oxides, or some other conductive andtransparent material. The first transparent layer is typically about 0.4to about 0.7 microns in thickness, more preferably about 0.45 to about0.5 microns in thickness. Suitable methods for forming the firstconductive layer include spray pyrolysis or chemical vapor deposition(CVD). The first transparent layer is suitably applied directly onto thesubstrate. A preferred first layer used in this invention is a layer oftin oxide applied to a glass substrate by CVD and further comprisingfluorine dopant to decrease the resistivity of the first transparentlayer to the desired resistivity. Preferably, the first transparentlayer has a resistivity of about 1×10⁻⁴ to about 5×10⁻⁴ ohm·cm. Flatglass substrates containing the first conductive layer comprising tinoxide doped with fluorine can be purchased from suitable glassmanufacturers and suppliers.

The second transparent layer of the thin film semiconductor device ofthis invention is a thin layer having a resistivity more than theresistivity of the first transparent layer. By thin we mean, preferably,the layer has a thickness of up to about 0.075 microns, or up to about0.065 microns, or up to about 0.05 microns, more preferably about 0.01to about 0.05 microns, and still more preferably of about 0.015 to about0.035 microns. Preferably, the thin, second transparent layer is atleast about 0.01 microns in thickness. Preferably, the secondtransparent layer comprises one or more conductive metal oxides such astin oxide, zinc oxide, indium tin oxide, zinc stannate, a mixture of oneor more of the above, or some other conductive and transparent material.Preferably, the second transparent layer comprises a metal oxide and is,preferably, the same metal oxide used in the first conductive layer butis doped differently so that its more resistive than the firsttransparent layer. Preferably, the second transparent layer comprises amixture of tin and zinc oxides. The molar ratio of tin oxide to zincoxide in the mixture is suitably about 99.9:0.1 to about 95:5,preferably about 99.5:0.5 to about 98:2, and more preferably about99.2:0.8 to about 98.8:1.2. The second transparent layer is preferablydeposited next to and in immediate contact with the first transparentlayer. The method used for depositing the second transparent layer isany suitable method that will provide the thin film of secondtransparent layer. For example, CVD, spray pyrolysis or physical vapordeposition can be used to form or deposit the second transparent layer.However, we discovered that the preferred method to form and deposit thesecond transparent layer is to use reactive direct current (DC)sputtering. In this process, a metal or a mixture of metals, for exampleelemental tin, elemental zinc, or mixtures thereof which may be inatomic ratios the same as the molar ratios mentioned above for tin oxideand zinc oxide, is sputtered onto the substrate. lndium can also be usedalone or in combination with either or both tin and zinc. In thepreferred method, such a mixture of metals is sputtered directly ontoand in direct contact with the first transparent layer. In the reactiveDC sputtering method, the metal or metals, for example, a mixture ofelemental tin and elemental zinc in an atomic ratio of about 99:1,respectively, are sputtered onto the substrate using, for example, anAirCo Thin Film Coater in an oxidizing atmosphere at a temperature ofabout 30° C. to about 90° C. and at a pressure of about 0.1 millitorr toabout 5 millitorr. The oxidizing atmosphere can be any atmosphere thatwill serve to convert the sputtered metal or metals to oxides during thesputtering process. For example, the oxidizing atmosphere can be amixture of oxygen gas with one or more inert gasses such as argon,helium or nitrogen such that the ratio of oxygen gas to inert gas isabout 99:1 to about 1:99. The oxidizing atmosphere can be oxygen gas.After the sputtering, the layer deposited is optionally heated in air oran oxidizing atmosphere, such as the oxidizing atmosphere describedabove, suitably at about 400° C. to about 500° C. for about 5 to about60 minutes, more preferably about 10 to about 30 minutes. Preferablyall, or substantially all, of the sputtered metal in the layer is in itsoxide form. By “substantially all” we mean, preferably, that at leastabout 98 percent, more preferably at least about 99 percent and mostpreferably at least about 99.9 percent of the metal in the depositedlayer is in the oxide form. We determined that the use of reactive DCsputtering as the method for depositing the second transparent layerprovides for a more efficient photovoltaic device of this inventioncompared, for example, to the same device where the second transparentlayer is applied by sputtering the metal followed by oxidation of thesputtered metal to the oxide by heating in air. Photovoltaic devices ofthis invention having a second transparent layer made by reactive DCsputtering a mixture of tin and zinc metal were also superior tophotovoltaic devices of this invention having a second transparent layermade by chemical vapor deposition of tin oxide or sputtered tin oxide.

The second transparent layer has a resistivity more than the resistivityof the first transparent layer. For example, the resistivity of thesecond transparent layer is up to about 1×10⁶ times more resistive,preferably about 1×10⁴ to about 5×10⁵ times more resistive than thefirst transparent layer. In absolute terms, the resistivity of thesecond transparent layer is suitably up to about 100 ohm·cm., preferablyabout 1 to about 15 ohm·cm, and more preferably about 5 to about 10ohm·cm. The transparency of the second transparent layer is suitably atleast about 90 percent, preferably at least about 95 percent and morepreferably at least about 99 percent, as measured by spectrophotometricmethods. The second transparent layer may be textured. Such texturingmay be accomplished by, for example, modulating the deposition rate ofthe metal oxides, etching after deposition with an acid such ashydrofluoric or hydrochloric acid, or with a base such as sodiumhydroxide, or by plasma or reactive ion etching. Although the secondtransparent layer in the invented semiconductor device may becrystalline or polycrystalline, it may be at least partially amorphous,for example, at least about 50 weight percent amorphous. Preferably itis amorphous or substantially amorphous. By substantially amorphous wemean, preferably, that the layer is at least about 95 weight percentamorphous, more preferably at least about 98 weight percent amorphousand most preferably at least about 99 weight percent amorphous. Althoughthe second transparent layer has been described hereinabove as aseparate layer, it is to be understood that the second layer can becontinuous with the first layer in that instead of a discrete boundarybetween the first layer and second layer there is a continuous or gradedchange in composition so as to achieve the purposes of the firsttransparent layer and second transparent layer as described herein.Thus, for example, rather than depositing a first transparent layer thenswitching to deposit the second transparent layer, the composition ofthe feedstock used to deposit the first layer, for example, tin or tinoxide, can be switched during the deposition process for example, to amixture of tin and zinc or tin oxide and zinc oxide, so that althoughthere may not be a discrete separation of the two layers, the regionsdeposited will function in the same manner as discrete layers.

The n-layer in the thin film semiconductor device of this invention isany suitable material that provides for compatibility with and capableof forming a heterojunction with a suitable p-layer. Preferably, then-layer forms a heterojunction with a p-layer, preferably a CdTep-layer, that will generate electrical charge separation upon exposureto light energy, preferably solar light energy. Preferably, the n-layer,also known as the window layer, comprises cadmium sulfide (CdS), but itcan also comprise zinc sulfide, cadmium zinc sulfide, or one or moremixtures of any of the above. The CdS layer can be deposited by anysuitable method. For example, it can be deposited by contacting thesubstrate (having the second transparent layer deposited on the firsttransparent layer) with an alkaline aqueous medium comprising a sourceof cadmium ion, sulfide ion (or precursor thereof), a colloid stabilizerand, preferably, a complexing agent for cadmium ion. The cadmium sourceis usually a water-soluble salt or complex, for example, an inorganicsalt such as cadmium chloride or sulfate, or organic salt, for example acarboxylate such as cadmium acetate. The aqueous medium may containabout 0.3 to about 10, preferably about 0.5 to about 2.5, and morepreferably about 1 to about 5 grams per liter of cadmium. The sulfide isusually a hydrosulfide ion or preferably an inorganic or organicprecursor thereof. The precursor, if used, is preferably water soluble,for example, to an extent of at least about 1 or preferably at leastabout 10 grams per liter. Examples of inorganic sulfide are hydrogensulfide, metal sulfides including alkali metal sulfide such as sodiumsulfide, alkaline earth metal sulfide such as calcium sulfide, or anonmetal or insoluble metal sulfide such as phosphorus pentasulfide oraluminum sulfide, respectively. Preferably the precursor is organic andis hydrolyzable, especially under alkaline conditions to give thesulfide ion. Examples are thiocarbonyl compounds such as thio-ketonesand aldehydes such as thioformaldehyde and thioacids and amides thereofespecially thio acids and amides, in particular thioacetamide andthiourea, as well as thiolacids (RC=O(SH). The aqueous solution maycontain about 0.01 to about 20, preferably about 0.05 to about 5, andmore preferably about 0.1 to about 1 gram of sulfide or sulfideprecursor (expressed by weight as sulfur) per liter of aqueous medium.

The colloid stabilizer is a material added to a colloidal dispersion tostabilize the dispersion. Such stabilizers are known in the generalfield of colloid chemistry. The colloidal stabilizer may be an inorganicsalt of a complex polyacid such as one of Group VA, VB or VIA of thePeriodic Table of the Elements, such as phosphorus, molybdenum ortungsten, for example, a polyphosphate or a heteropolyacid. Sodium metalphosphate and sodium tripolyphosphate are preferred. The colloidalstabilizer may also be a polymeric water soluble hydrophilic compound,such as a synthetic polymer, for example, polyvinyl alcohol orpoly(meth)acrylic acid or polyvinyl pyrrolidone, or a natural polymer,such as vegetable gum, for example, guar gum, or gelatin or xanthan gum.The colloidal stabilizer may be present in the aqueous medium in amountsof about 0.01 to about 30, preferably about 0.05 to about 20, and morepreferably about 0.05 to about 5 grams per liter of the aqueous medium.

The complexing agent, which is preferably present in the aqueous medium,complexes the cadmium ion under the pH conditions in the medium. Thecomplexing agent is usually water soluble and, preferably, nitrogenous,preferably with at least one amino nitrogen atom, for example 1 to about4 amino nitrogen atoms, and may be added as free base or as an aminesalt. Thus, the complexing agent may be ammonia, usually added as anammonium salt, for example a halide such as a chloride, a nitrate or asulphate. It may also be a halide, nitrate or sulfate of a primary,secondary or tertiary organic amine and, in particular, of an aliphaticamine such as an amino alkane, which may also have at least one hydroxylsubstituent. Examples of such organic amines are mono-,di- andtriethanolamine and propanolamine. Other suitable amines are alkylenediamines such as ethylene diamine, and amino acids such as ethylenediamine tetracetic acid. The concentration of complexing agent in theaqueous medium is usually about 10 to about 200, preferably about 40 toabout 100 grams per liter of aqueous medium. In particular, the ratio ofmoles of complexing agent to Cd atoms is usually about 1:1 to about1000:1, for example, about 50:1 to about 500:1.

The aqueous medium usually has a pH of about 8 to about 13, preferablyabout 9 to about 12.5, which may be achieved by addition of alkali, forexample, sodium hydroxide, a basic complexing agent such as ammonia, orboth.

The deposition process is usually performed at a temperature of about 20to about 90° C., preferably about 50 to about 80° C., and in a time ofabout 5 to about 100 minutes, preferably about 10 to about 50 minutes.The aqueous medium may be unagitated or may be agitated eitherperiodically or continuously. The process may be performed by mixing allthe components and allowing reaction to occur, but preferably thestabilizer and cadmium ion, optionally with complexing agent, are mixedfirst to give a solution thereof and then to this is added the sulfideor precursor thereof. The preferred method to form a CdS layer for thethin film semiconductor of this invention is to use a chemicaldeposition process where the CdS is deposited by immersing the substratecontaining the first and second transparent layer deposited thereon in abath containing a warm alkaline solution containing the cadmium complex([Cd(NH₃)₄]²⁺ and thiourea. Such processes are disclosed, for example,in N. R. Pavaskar, et al., J. Electrochem. Soc. 124 (1967) p. 743, andin I. Kaur, et al., J. Electrochem. Soc. 127 (1981) p. 943 which areboth incorporated herein by reference. Irrespective of the method used,the CdS layer can be deposited to a thickness of up to about 0.12microns. After deposition of the CdS layer, it is typically heated inair at a temperature of about 300° C. to about 500° C., preferably atabout 350° C. to about 450° C. for about 10 to about 60 minutes, morepreferably about 15 to about 40 minutes, to anneal the CdS layer. Duringthis annealing process it is believed that the CdS layer undergoesdensification and grain growth.

We discovered that the CdS layer in the thin film semiconductor deviceof this invention can be made much thinner than the CdS layers in priorphotovoltaic devices containing CdTe and CdS layers. For example, in thethin film semiconductor device of this invention the CdS layer orn-layer can be thin. By thin we mean, preferably, up to about 0.07microns in thickness, more preferably about 0.01 to about 0.065 micronsin thickness, and most preferably about 0.04 to about 0.06 microns inthickness. Such thin CdS layers are desirable because the CdS layer,although a window layer, nevertheless absorbs light energy which webelieve contributes to the lowering of the efficiency of a CdS/CdTephotovoltaic device. Consequently, the thinner CdS layer or n-layer ofthe thin film semiconductor device of this invention when used in aphotovoltaic device results in a more efficient photovoltaic device forconverting light energy into electrical energy.

The p-layer in the thin film semiconductor device of this inventionpreferably comprises one or more IIB elements and one or more VIAelements of the Periodic Table of Elements as appearing in “AdvancedInorganic Chemistry” by Cotton and Wilkinson, 4^(th) Edition, in whichthe Group IIB elements include cadmium and the Group VIA elementsinclude selenium and tellurium. The preferred p-layer comprises cadmiumand tellurium which may also contain mercury as disclosed in U.S. Pat.No. 4,548,681 which is incorporated herein by reference. Additionally,the p-layer may contain quantities of dopants such as one or more ofcopper, gold or silver as disclosed in EP Patent 244963 which isincorporated herein by reference. Other p-type layers include, forexample, copper indium diselenide, copper sulfide, copper indiumdisulfide, GaSb, GaAs, Sn1_(±x)Se, InSb, CulnSe_(2−x), and along withCdTe one or more mixtures thereof, or one or more of the p-type layersdisclosed in U.S. Pat. No. 4,753,684 which is incorporated herein byreference. However, the preferred p-layer comprises Cd and Te,preferably CdTe, with or without mercury or the dopants mentioned above.Preferably the CdTe p-layer is deposited by electrodeposition. Asuitable method for electrodeposition of a CdTe layer as well as othersuitable IIB and VIA elements is disclosed in Panicker, et al.,“Cathodic Deposition of CdTe from Aqueous Electrolytes,” J. Electrohem.Soc. 125, No. 4, 1978, pp. 556-572, and in U.S. Pat. No. 4,400,244 whichare both incorporated herein by reference. In this method, deposition ofCdTe takes place from an aqueous solution of CdSO₄ to which TeO₂ hasbeen added and the electrodeposition is carried out onto the substratehaving the first and second transparent layers, and the CdS layerdeposited thereon. Preferably, in the solution the concentration of Cd²⁺ions is about 0.2 to about 1.5 molar, and the concentration of HTeO₂+ions is about 10⁻⁵ molar to about 10⁻³ molar, and the pH of the solutionis suitably about 1 to about 3, and is conveniently adjusted by an acidsuch as sulfuric or hydrochloric acid. In such an electrochemicalmethod, HTeO₂+ at the cathode reacts with Cd²+ ions to form cadmiumtelluride which is deposited on the glass substrate cathode. In thesemiconductor device of this invention, the p-layer having IIB and VIAelements, for example, a p-layer of CdTe, copper indium diselenide,GaSb, GaAs, Sn_(1±x)Se, InSb, CulnSe_(2−x) or one or more mixturesthereof, and particularly a CdTe p-layer, is preferably deposited byelectrodeposition directly onto and in direct contact with the CdSwindow layer. Methods for the electrodeposition of suitable p-layers orprecursors therof useful in this invention are also disclosed in EPPatent 0538041 which is incorporated herein by reference.

A CdTe layer deposited electrolytically as described above has n-typeconductivity and, therefore, cannot form a rectifying heterojunctionwith a CdS n-type layer capable of generating electrical energy uponexposure to light renergy. To produce a rectifying junction, the n-typeCdTe layer is heat treated in air at, for example, a temperature ofabout 250 to about 500° C. for a time sufficient, for example, about 5to about 10 minutes, to convert the n-type CdTe layer to a relativelylow resistivity p-type layer. Such a heating process is disclosed inU.S. Pat. No. 4,388,483 which is incorporated herein by reference.

The p-type layer in the thin film semiconductor device of this inventionis typically up to about 5 microns in thickness, preferably about 0.5microns to about 3.0 microns in thickness, and more preferably about 1.5to about 2.5 microns in thickness. During the heat treatment of the CdTelayer as described above, the CdTe layer preferably recrystallizes andundergoes grain growth.

The semiconductor devices of this invention, if used as a photovoltaicdevice, generally have a back contact. This back contact is preferablydeposited on and in direct contact with the p-layer. The back contact issuitably made from one or more highly conductive materials. Theconductive back contact may be, for example, one or more of elementalnickel, chromium, copper, tin, aluminum, gold, silver, technecium oralloys or mixtures of any of the above such as, for example, an alloy oftin and zinc. It can be layers of one or more metals such as the metalsjust mentioned, for example a layer of nickel and a layer of chromium.It can be made from blends of graphite and polymeric materials, carbonpastes and, it can also be a transparent conductive oxide such as, forexample, the conductive oxides described hereinabove useful for thefirst transparent layer. The back conductive contact can be a layer ofcarbon deposited on the p-layer followed by one or more layers of metal,such as the metals described above. The back conductive contact, if madeof or comprising one or more metals, is suitably applied by a techniquesuch as sputtering or metal evaporation. If it is made from a graphiteand polymer blend, or from a carbon paste, the blend or paste is appliedto the semiconductor device by any suitable method for spreading theblend or paste, such as screen printing, spraying or by a “doctor”blade. After the application of the graphite blend or carbon paste, thedevice is heated to convert the blend or paste into the conductive backcontact layer. Suitable carbon-containing pastes or inks can beobtrained from suppliers such as DuPont Microcircut Materials, MettechPolymers Group, Acheson Colloids Company, and Coates Circuit Products.Suitable back contacts are disclosed in U.S. Pat. No. 4,735,662 which isincorporated herein by reference. A carbon layer, if used, is suitablyabout 1 to about 10 microns in thickness. A metal layer of the backcontact, if used for or as part of the back contact, is suitably about0.1 to about 1 microns in thickness. Prior to adding the back conductivecontact, the p-layer may be treated as set forth in U.S. Pat. Nos.4,456,630 and 5,472,910 which are incorporated herein by reference.These references teach methods to dope the p-layer and methods toimprove the ohmic contact between the p-type layer and the conductiveback contact.

When using the semiconductor device of this invention as a photovoltaicdevice, it is useful to connect a plurality of the devices in series inorder to achieve a desired voltage. Any suitable method can be used toaccomplish such a connection, for example, electrical wiring or otherconductive means can be used to connect a plurality of devices inseries. Each end of the series connected cells can be attached to asuitable conductor such as a wire or bus bar, to direct thephotovoltaically generated current to convenient locations forconnection to a device or other system using the generated electric. Aconvenient means for achieving such series connections is to laserscribe the device to divide the device into a series of cells connectedby interconnects. Methods for interconnecting cells in a seriesconfiguration are disclosed in U.S. Pat. Nos. 4,243,432 and 4,383,022which are incorporated herein by reference. Preferably, a laser is usedto scribe the deposited layers of the semiconductor device to divide thedevice into a plurality of series connected cells. A laser scribingprocess is disclosed in the article by D. Cunningham et al., “Large AreaApollo Module Performance And Reliability,” 28^(th) IEEE PhotovoltaicSpecialists Conference, Anchorage, Ak. September 2000, which isincorporated herein by reference.

One embodiment of the invention will now be described with reference toFIG. 1.

FIG. 1 shows the layered structure of one embodiment of a thin film,layered semiconductor 1 of this invention. The layers are not drawn toscale in FIG. 1 with respect to the relative thickness of each layer.

In FIG. 1, 2 is a glass substrate, preferably a flat glass substratemade from a high quality float glass. Layer 3 is a first transparentconductive layer comprising tin oxide doped with fluorine atoms to makeit conductive. Layer 3 has a thickness of about 0.5 microns and ispreferably deposited on the glass substrate by CVD. Alternatively, glasssubstrate 2 can be purchased from glass suppliers having the layer 3deposited thereon. Layer 4 is a second transparent layer having aresistivity more than the resistivity of conductive layer 3. Layer 4,for example, has a resistivity of about 1 to about 100 ohm·cm. Layer 4is a mixture of tin and zinc oxides in a molar ratio of 99:1 formed byreactive DC sputtering and is about 0.03 microns in thickness.

Layer 5 is a CdS n-type or window layer formed by chemical depositionfrom a bath of a warm alkaline solution of cadmium complex ([Cd(NH₃)₄]²⁺and thiourea. The deposited CdS is heated in air at about 400° C. forabout 30 minutes after deposition. Layer 6 is a p-type layer of CdTeformed by electrodeposition from an acidic bath of Cd²⁺ and TeO₂. Afterelectrodeposition, the deposited CdTe layer is heated or annealed in airat a temperature of about 450° C. for about 20 minutes to convert it tothe desired p-layer. Upon such heating, the photovoltaically activeheterojunction depicted by line 7 is formed between the n-type, CdSlayer and the p-type CdTe layer. Layer 8 is a conductive back contactmade of carbon formed by applying and heating a carbon-containing ink.Layer 9 is a layer of metal, such as tin, added by sputtering and isabout 0.25 to about 0.75 microns in thickness.

Although not wishing to be bound by any theory of operation, we believethat the thin, second transparent layer in the semiconductor device ofthis invention, and in the photovoltaic device made therefrom, reduceselectrical shunting of what would otherwise occur between the CdTep-layer and the first transparent layer. In prior devices, defects inthe CdS n-layer may cause such shunting. In order to reduce theshunting, the thickness of the CdS layer was typically increased.However, upon increasing the thickness of the window CdS layer, there isa concomitant decrease in the efficiency of the photovoltaic device dueto the absorption of light by the thick CdS layer. In the semiconductordevice of this invention, the thin second transparent layer is believedto alleviate such shunting because of its lower conductivity or higherresistivity. Thus, even though the thinner CdS layer may still havedefects, the shunting is reduced or eliminated by the presence of thethin, higher resistivity second transparent layer.

In the semiconductor device of this invention, the second transparentlayer, although having a conductivity less than the conductivity of thefirst transparent layer, has a conductivity preferably sufficiently highto permit the electrodepositon of a CdTe layer as described hereinabove.For example, if the resistivity of the second transparent layer is toolarge, for example, more than about 15 ohm·cm, or more than about 10ohm·cm, a CdTe layer deposited by the electrochemical methods asdescribed herein, for example, may be non-stoichiometric. Bynon-stoichiometric we mean it will not have the preferred Cd:Te atomicratio. A non-stoichiometric CdTe layer may lead to a less than optimalCdTe absorber layer for conversion of solar energy or other light energyto electrical energy. Thus, the second layer is preferably of athickness and resistivity to reduce or eliminate shunting associatedwith thin CdS layer yet of a sufficient conductivity to provide for theprefered stoichiometry of the CdTe layer formed by an electrolyticdeposition process. By prefered stoichiometry we mean, preferably, astoichiometry where the atomic ratio of Cd to Te is within the rangeCd_(1+x)Te_(1+y), where x and y are no more than about ±0.01. Stateddifferently the CdTe stoichiometry should be within 1% of the 1:1stoichiometry. Preferably the atomic ratio of Cd to Te is substantially1:1, and most preferably the atomic ratio of Cd to Te is 1:1 in the CdTelayer of the semiconductor device of this invention.

We have also found the thin second transparent layer of thesemiconductor device of this invention provides for an improvedphotovoltaic device, e.g., a device with higher efficiency, compared toa photovoltaic device having a thicker second transparent layer.Photovoltaic devices of this invention comprising the semiconductordevice of this invention have an improved Isc without loss of opencircuit voltage (Voc) and fill factor (FF) compared to prior artCdS/CdTe photovoltaic devices.

This invention is also a method for making the thin film semiconductordevices of this invention. The method comprises depositing on asubstrate, suitably a glass substrate, a first transparent layer asdescribed hereinabove comprising a transparent conductive material asdescribed hereinabove; depositing a second transparent layer asdescribed hereinabove having a conductivity less than the conductivityof the first transparent layer; depositing an n-type, preferably thin,layer as described hereinabove; depositing a p-type layer as describedhereinabove or, as also described hereinabove, a layer which can beconverted into a p-type layer after deposition; and depositing a second,generally opaque, conductive layer or layers as described above whichcan serve as an electrical contact. Photovoltaic devices comprising thethin film semiconductor devices of this invention are highly efficientin converting light energy into electrical energy. For example,photovoltaic devices of this invention comprising the thin layersemiconductor devices of this invention have efficiencies of at leastabout 8.5 percent, of at least about 9.0 percent, of at least about 9.5,of at least about 10, or of at least about 10.5 percent. Efficiencies ofat least about 11 or about 11.5 percent can be achieved. Photovoltaicdevices comprising the semiconductor devices of this invention haveefficiencies of about 11.5 percent to about 8.5 percent, for exampleabout 11 percent to about 9 percent. The efficiency of a photovoltaicdevice of this invention made using the semiconductor device of thisinvention can be conveniently and preferably is measured according toASTM E-948-95.

The following examples are being provided to illustrate certainembodiments of the invention, however, they are not intended to limit inanyway the scope thereof.

Provisional patent application 60/289,481 filed on May 8, 2001, isincorporated by reference in its entirety. All references to thePeriodic Table of Elements hereinabove are to the Periodic Table ofElements as appearing in “Advanced Inorganic Chemistry,” Cotton andWilkinson, 4^(th) Ed.

EXAMPLES

Thin film photovoltaic devices were made as follows and tested forefficiency in converting light energy to electrical energy. Thephotovoltaic devices tested had a 3 mm thick float glass sheet as asubstrate material. The substrate was coated with a layer of transparentconducting tin oxide about 0.5 to 0.6 microns thick. The conductive tinoxide was applied using chemical vapor deposition and had a resistivityof about 1×10⁻⁴ ohm·cm. A second transparent layer of tin and zinc oxidein a molar ratio of 99 to 1 was deposited on the first transparent layerusing reactive DC magnetic sputtering in an oxygen atmosphere at apressure of about 1 millitorr. The thickness of the second transparentlayer was varied as shown in the Table 1. After deposition, the secondtransparent layer was heated in air at a temperature of 500° C. for 20minutes. The second transparent layers had resistivities that were aboutthe same. A 0.05 microns film of cadmium sulfide was deposited on thesecond transparent layer by chemical deposition by the reaction ofcadmium ion with thiourea in an aqueous ammonia solution at atemperature of about 70° C. The cadmium sulfide layer was heated in airat 400° C. for 30 minutes after deposition. A layer of cadmium telluridewas deposited on the cadmium sulfide layer by electrodeposition from abath of cadmium sulfate and tellurium dioxide. Chloride ion was alsopresent and was incorporated in the cadmium telluride layer during theelectrodeposition process. The electrodeposited cadmium telluride layerwas about 1.8 microns in thickness after deposition. After deposition,the substrate containing the deposited layers was heated in air at 450°C. for 15 minutes. After heat treatment, the cadmium telluride layer wasdoped with copper at 200° C. A carbon layer was applied to thecopper-doped cadmium telluride by screen printing a layer of carbon ink,and the ink was heated at 100-200° C. in air to form a carbon layerabout 10 microns in thickness. The carbon layer was then coated withaluminum by sputtering a 0.3 microns thick layer of aluminum.

Table 1 reports the efficiency of the photovoltaic devices so formed.

TABLE 1 Thickness of Second Transparent Layer^(a) % Efficiency 0.10 2.40.075 5.1 0.051 7.5 0.025 8.9 ^(a)= microns

The results in Table 1 show an increase in efficiency of the thin filmphotovoltaic devic as the second transparent layer is made thinner.

Only certain embodiments of the invention have been set forth andalternative embodiments and various modifications will be apparent fromthe above description to those of skill in the art. These and otheralternatives are considered equivalents and within the spirit and scopeof the invention.

1. A method for making a thin film semiconductor device suitable for usein a photovoltaic device, the method comprising (a) forming on asubstrate a first transparent layer comprising a conductive material,(b) forming a second transparent layer up to about 0.075 microns inthickness comprising tin oxide and zinc oxide and where the mole ratioof tin oxide to zinc oxide is about 99.9:01 to about 95:5 and having aresistivity of the first transparent layer, (c) forming an n-type layeror precursor layer thereof, and (d) forming a p-type layer.
 2. Themethod of claim 1 wherein the second transparent layer is is in directcontact with the first transparent layer.
 3. The method of claim 2wherein the n-type layer is in direct contact with the secondtransparent layer.
 4. The method of claim 3 wherein the p-type layer isin direct contact with the n-type layer.
 5. The method of claim 1wherein the second layer is about 0.01 to about 0.05 microns inthickness.
 6. The method of claim 1 wherein the second transparent layeris deposited using reactive DC sputtering.
 7. The method of claim 1wherein the n-type layer comprises cadmium sulfide.
 8. The method ofclaim 7 wherein the cadmium sulfide n-type layer is up to about 0.07microns in thickness.
 9. The method of claim 1 wherein the secondtransparent layer has a resistivity that provides for the deposition ofa CdTe layer in a stoichiometry of about 1:1 by electrochemicaldeposition.
 10. A method for making a thin film semiconductor devicesuitable for use in a photovoltaic device, the method comprising (a)forming on a substrate a first transparent layer comprising a conductivematerial, (b) forming a thin second transparent layer having aresistivity more than the restivity of the first transparent layer, (c)forming an n-type layer, and (d) forming a p-type layer, and wherein thethin second transparent layer is formed by reactive DC sputtering atarget consisting essentially of metal in an oxidizing atmosphere. 11.The method of claim 10 wherein the thin second transparent layercomprises tin oxide.
 12. The method of claim 10 wherein the thin secondtransparent layer comprises zinc oxide.
 13. The method of claim 10wherein the target comprises a mixture of tin and zinc.